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Researchers have found that smells, particularly unpleasant ones, leave their mark in the brain when smelt for the first time.

The study sheds light on why smells sometimes bring back memories and could be used to assist people overcoming trauma.

The researchers, led by Yaara Yeshurun of the Weizmann Institute of Science in Israel, displayed a set of images to a group of volunteers along with pleasant and unpleasant sounds (guitar and electric drill) and smells (pear and fungus) - known as the exposure task.

The volunteers were then placed in a functional magnetic resonance imaging (fMRI) scanner and were asked to recall the sound or smell associated with each image.

After 90 minutes they repeated the exposure task, which associated different sounds and smells to each image, and were again placed in the fMRI scanner.

Eight days later, the volunteers entered the fMRI scanner, to test which sound and smell they most associated with each image shown.

The researchers found that the volunteers were more likely to remember the associations that contained unpleasant stimuli (50.5%) compared to those with pleasant stimuli (34.5%).

For example, if the volunteer was shown a picture of a horse and they smelt the pear during the first test and the fungus during the second, they were more likely to recall the fungus smell when shown the horse eight days later.

But the researchers also found that first time smells (pleasant and unpleasant) noticeably activated the left hippocampus and the right amygdala - areas in the brain associated with learning, memory and fear conditioning.

This suggests that people are more likely to recall the first smell associated with an event and may explain why some smells trigger childhood memories, say the reseachers.

The effect was so clearly defined that the researchers were able to predict how well a volunteer recalled smells eight days later, based on their first fMRI scans.

Risk management

According to Yeshurun and colleagues it makes good sense to remember unpleasant memories as a kind of evolutionary "risk management".

"[It] may represent a potential adaptive mechanism considering the potential cost of failing to learn a first negative association and the potential benefit of a malleable first positive association," they write.

Professor John Rostas, a neurochemist at the University of Newcastle, says the study provides an objective approach to the study of smell and memory, something many of us have suspected in non-scientific literature.

"Early experiences of smells are very strongly associated with visual images or emotions," he says.

"New techniques of brain imaging seem to be yielding results that mesh very well with experience."

Incomplete

But Emeritus Professor Robert Gregson of the Australian National University in Canberra, who has previously conducted research into olfactory senses, believes the study is "incomplete".

"I don't think the case they have outlined is strong," he says.

Gregson says our olfactory senses are "remarkably thin" when it comes to detecting intensity. He adds that some people have trouble detecting smells that seem quite commonplace.

He also believes that what is considered an unpleasant smell to some people, is the opposite to others.

"There is data showing that children do not respond to nice or nasty smells in the same way that adults do, and so nice and nasty don't always mean the same way."

Gregson believes the study needs to encompass a broader range of people from various backgrounds.

Any application of the findings would be far off, says Yeshurun, but she suggests the results could be used to help with post traumatic stress.

"It may help us generate methods to better forget early and powerful memories, such as trauma."